Polishing pad for chemical mechanical polishing

ABSTRACT

The present invention provides in one embodiment, a polishing pad  100  for chemical mechanical polishing. The polishing pad comprises a polishing body  110 . The polishing body comprises a thermoplastic foam substrate  115  having a surface  120  comprising concave cells  125 . A polishing agent  130  coats an interior surface  135  of the concave cells. The polishing agent comprises an inorganic metal oxide that includes carbides or nitrides. Yet another embodiment of the present invention is a method for preparing a polishing pad  200.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application claims the benefit of U.S. Provisional Application Ser.No. 60/503,152 filed on Sep. 15, 2003, entitled “CORROSION RETARDINGPOLISHING SLURRY FOR THE CHEMICAL MECHANICAL POLISHING OF COPPERSURFACES,” commonly assigned with the present invention and incorporatedherein by reference which is a continuation-in-part of U.S. applicationSer. No. 10/241,074, entitled, “A POLISHING PAD SUPPORT THAT IMPROVESPOLISHING PERFORMANCE AND LONGEVITY,” to Yaw S. Obeng and Peter Thomas,filed on Sep. 11, 2002, which in turn, is a continuation in part of U.S.Pat. No. 6,579,604 entitled, “A METHOD OF ALTERING AND PRESERVING THESURFACE PROPERTIES OF A POLISHING PAD AND SPECIFIC APPLICATIONSTHEREFOR,” to Yaw S. Obeng and Edward M. Yokley, filed on Nov. 27, 2001,and a continuation-in-part of and of U.S. patent application Ser. No.10/241,985 entitled, MEASURING THE SURFACE PROPERTIES OF POLISHING PADSUSING ULTRASONIC REFLECTANCE, to Yaw S. Obeng, filed on Sep. 12, 2002and incorporated by reference as if reproduced herein in their entirety.

TECHNICAL FIELD OF THE INVENTION

The present invention is directed to chemical mechanical polishing forcreating a smooth, ultra-flat surface on such items as glass,semiconductors, dielectrics, metals and composites thereof, magneticmass storage media and integrated circuits.

BACKGROUND OF THE INVENTION

Chemical mechanical polishing (CMP) has been successfully used forplanarizing both metal and dielectric films. In one plausible mechanismof planarizing, the polishing process is thought to involve intimatecontact between high points on the wafer surface and the pad material,in the presence of slurry. In this scenario, corroded materials,produced from reactions between the slurry and wafer surface beingpolished, are removed by shearing at the pad-wafer interface. Theelastic properties of pad material significantly influence the finalplanarity and polishing rate. In turn, the elastic properties are afunction of both the intrinsic polymer and its foamed structure.

Historically, polyurethane-based pads have been used for CMP because oftheir high strength, hardness, modulus and high elongation at break.While such pads can achieve both good uniformity and efficienttopography reduction, their ability to rapidly and uniformly removesurface materials drops off rapidly as a function of use. The drop offin material removal rates as a function of time observed forpolyurethane-based pads has been attributed to changes in the mechanicalresponse of such polishing pads under conditions of critical shear. Itis generally believed that the loss in functionality ofpolyurethane-based CMP pads is due to pad decomposition from theinteraction between the pad and the slurries used in the polishing.

Moreover, decomposition produces a surface modification in and of itselfin the case of the polyurethane pads which can be detrimental to uniformpolishing. Alternatively, in some instances, the surface modification ofmaterials used for CMP polishing pads may improve the applicationperformance. Such modifications, however may be temporary, thusrequiring frequency replacement or retreatment of the CMP pad.Polyurethane pads also generally require a break-in period beforepolishing, in addition to the reconditioning and retreatment after aperiod of use. It is often also necessary to keep traditional pads wetin while polishing equipment is in idle mode. These characteristicsundesirably reduce the overall efficiency of CMP when using polyurethaneor similar conventional pads.

Accordingly, what is needed is an improved CMP pad capable of providinga highly planar surface during CMP and having improved longevity, whilenot experiencing the above-mentioned problems.

SUMMARY OF THE INVENTION

To address the above-discussed deficiencies of the prior art, thepresent invention provides in one embodiment, a polishing pad forchemical mechanical polishing. The polishing pad comprises a polishingbody comprising a thermoplastic foam substrate. The thermoplastic foamsubstrate has a surface comprising concave cells. A polishing agentcoating an interior surface of the concave cells comprises an inorganicmetal oxide that includes carbides or nitrides.

Another embodiment of the present invention is directed to a method forpreparing a polishing pad. The method comprises exposing closed cellswithin a thermoplastic foam substrate to provide a substrate surfacecomprising concave cells. The method further includes coating aninterior surface of the concave cells with a polishing agent comprisingan inorganic metal oxide, wherein carbides and nitrides are incorporatedinto the inorganic metal oxide during the coating.

The foregoing has outlined preferred and alternative features of thepresent invention so that those skilled in the art may better understandthe detailed description of the invention that follows. Additionalfeatures of the invention will be described hereinafter that form thesubject of the claims of the invention. Those skilled in the art shouldappreciate that they can readily use the disclosed conception andspecific embodiment as a basis for designing or modifying otherstructures for carrying out the same purposes of the present invention.Those skilled in the art should also realize that such equivalentconstructions do not depart from the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference isnow made to the following descriptions taken in conjunction with theaccompanying drawings, in which:

FIG. 1 illustrates a cross sectional view of a polishing pad of thepresent invention;

FIGS. 2-4 illustrate cross sectional views of selected step in a methodof the present invention for preparing a polishing pad;

FIG. 5 presents representative near infrared spectra of samples ofthermoplastic foam polishing pads after variable periods of coating witha polishing agent precursor comprising tetraethoxy silane (TEOS);

FIG. 6 illustrates changes in the near infrared signal forrepresentative thermoplastic foam polishing pads exposed to differentcoating periods with TEOS;

FIG. 7 illustrates exemplary indentation curves for a thermoplastic foampolishing pad after being coated with TEOS;

FIG. 8 illustrates the representative change in (Xa), ‘pop-in’ forthermoplastic foam polishing pads as a function of coating time withTEOS;

FIG. 9 illustrates the representative change in hardness forthermoplastic foam polishing pads as a function of coating time withTEOS;

FIG. 10 illustrates the representative change in elastic modulus forthermoplastic foam polishing pads as a function of coating time withTEOS;

FIG. 11 illustrates the representative change in storage and lossmodulus for thermoplastic foam polishing pads as a function of coatingtime with TEOS;

FIG. 12 presents representative XPS spectra of polishing pads aftervarious periods of coating time with TEOS;

FIG. 13 presents the change in the Oxygen to Si intensity ratio valuescalculated from the XPS spectra of polishing pads after various periodsof coating times with TEOS; and

FIG. 14 presents the change in relative Blanket Tungsten Removal Rate(WRR) and the Static Coefficient of Friction (COF) for thermoplasticfoam polishing pads as a function of coating time with TEOS.

DETAILED DESCRIPTION

The present invention benefits from the previously unrecognizedadvantages of using a thermoplastic polymer as the substrate fordepositing a uniform coating of a polishing agent on concave cells. Theinterior surface of the concave cells was discovered to form excellentreceptacles for receiving a uniform coating of the polishing agent. Itis hypothesized that the center of the concave cell serves as anexcellent nucleating point for coating because the surface energy of thecell at the center is lowest. It is believed that the initiation ofcoating at this location facilitates the uniform coverage of theinterior surface of the concave cell with the polishing agent, therebyfacilitating the polishing performance of a pad having such a surface.

The polishing agent of the present invention comprises an inorganicmetal oxide that includes nitrides or carbides. Using such metal-oxidesto coat the pad surface also advantageously makes the polishing padsurface permanently hydrophilic. Preferably, the inorganic metal oxidehas a lattice of atoms that incorporate the nitrides or carbides intothe lattice. The use of such polishing agent surface coatings enhancespolishing by modifying the surface mechanical properties of polishingpads.

For instance, altering the nitride or carbide content of particularinorganic metal oxides allows the mechanical properties of the polishingpad surface to be fine tuned so as to match the mechanical propertiesthe surface being polished. In turn, matching the mechanical propertiesof the polishing pad surface to the surface being polished improvespolishing rate selectivity and reduces process-induced defects, such asscratching. Tuning the surface mechanical properties is accomplished bytaking advantage of thermoplastic polymer substrate alterations producedby the plasma enhanced chemical vapor deposition (PECVD) surface coatingand the secondary thermal induced reactions in the bulk of thethermoplastic substrate.

One embodiment of the present invention is a polishing pad for chemicalmechanical polishing semiconductor devices. FIG. 1 presents an exemplarypolishing pad 100 of the present invention. The polishing pad 100comprises a polishing body 110. The polishing body 110 comprises athermoplastic foam substrate 115 having a surface 120 comprising concavecells 125. A polishing agent 130 coating an interior surface 135 of theconcave cells 125 comprises an inorganic metal oxide that includescarbides or nitrides.

The polishing agent 130 comprises ceramic compounds composed of one ormore inorganic metal oxides formed by the grafting of secondaryreactants on the thermoplastic foam substrate 115 surface 120 in aplasma-enhanced chemical vapor deposition (PECVD) process. As furtherexplained below, in the present invention, the PECVD process can bealtered to promote the inclusion of carbides or nitride in the inorganicmetal oxide.

It is preferable for one or both of the carbides or nitrides to beincorporated into a lattice of the inorganic metal oxides. For instance,when the inorganic metal oxide comprises a silicon oxide, then thelattice can comprise silicates in with polymeric Si—O—Si structureshaving tetrahedral and distorted tetrahedral configurations. Thenitrides can comprise silicon nitrides that are incorporated into theselattices. Alternatively, when the inorganic metal oxide comprises atitanium oxide, then the nitrides can comprise titanium nitridesincorporated into titanium oxide lattices. Similarly, silicon carbidesand titanium carbides can be incorporated into a polishing agent 130whose inorganic metal oxides comprise silicon oxide and titanium oxide,respectively. In some preferred embodiments, nitrides, such as siliconnitride, comprise about 10 mol percent of the polishing agent 130, whilein other embodiments, carbides, such as silicon carbide, comprise about10 mol percent of the polishing agent. In some preferred embodiments thepolishing agent 130 can comprise both nitride and carbides at theseconcentrations.

As the PECVD process is extended to longer periods, the silanolconcentration in the polishing agent 130 decreases, resulting in adecrease in the ratio of oxygen to silicon. In some advantageousembodiments of the polishing pad 100 where the polishing agent 130comprises silicon oxide, the O:Si ratio is at least about 8:1, and insome cases, at least about 9.9:1.

Including nitrides and carbides in the polishing agent 130 provides anadditional, heretofore unrecognized, means to alter the mechanicalproperties of the polishing pad 100, and thereby alter the pad'spolishing properties. In some preferred embodiments, the polishing pad100 has a hardness of greater than 60 KPa, and more preferably greaterthan 70 KPa. In other preferred embodiments, the polishing pad 100 has ahardness between about 62 KPa and about 70 KPa. In certain preferredembodiments, the polishing pad 100 has an elastic modulus of aboutgreater than about 3 MPa, and more preferably, 4 MPa or greater. Inother preferred embodiments, the polishing pad has an elastic modulus ofbetween about 4 MPa and 8.2 MPa, respectively. In yet other embodiments,the polishing pad 100 has a loss modulus of about 0.4 MPa or greater,and more preferably at least about 0.6 MPa.

As disclosed in U.S. Pat. No. 6,579,604 and U.S. application Ser. No.10/241,074, incorporated herein by reference, the inorganic metal oxideof the polishing agent 130 can be produced from a variety ofoxygen-containing organometallic compound used as the secondary reactantin a PECVD process. For example, the secondary plasma mixture mayinclude a transition metal such as titanium, manganese, or tantalum.However, any metal element capable of forming a volatile organometalliccompound, such as metal ester contain one or more oxygen atoms, andcapable of being grafted to the polymer surface is suitable. Silicon mayalso be employed as the metal portion of the organometallic secondaryplasma mixture. In these embodiments, the organic portion of theorganometallic reagent may be an ester, acetate, or alkoxy fragment. Inpreferred embodiments, the inorganic metal oxide of the polishing agent130 comprises silicon oxides or titanium oxides, such as silicon dioxideor titanium dioxide, respectively; tetraethoxy silane polymer; ortitanium alkoxide polymer.

Other secondary plasma reactants include ozone, alkoxy silanes, water,ammonia, alcohols, mineral sprits or hydrogen peroxide. In somepreferred embodiments, the secondary plasma reactant comprises titaniumesters; tantalum alkoxides, including tantalum alkoxides wherein thealkoxide portion has 1-5 carbon atoms; manganese acetate solution inwater; manganese alkoxide dissolved in mineral spirits; manganeseacetate; manganese acetylacetonate; aluminum alkoxides; alkoxyaluminates; aluminum oxides; zirconium alkoxides, wherein the alkoxidehas 1-5 carbon atoms; alkoxy zirconates; magnesium acetate; andmagnesium acetylacetonate. Other embodiments are also contemplated forthe secondary plasma reactant, for example, alkoxy silanes and ozone,alkoxy silanes and ammonia, titanium esters and water, titanium estersand alcohols, or titanium esters and ozone.

Some preferred embodiments of the thermoplastic foam substrate 115comprise cross-linked polyolefins, such as polyethylene, polypropylene,and combinations thereof. In certain preferred embodiments, thethermoplastic foam substrate 115 comprises a closed-cell foam ofcrosslinked homopolymer or copolymers. Examples of closed-cell foamcrosslinked homopolymers comprising polyethylene (PE) include: Volara™and Volextra™ from Voltek (Lawrence, Mass.); Aliplast™, from JMSPlastics Supply, Inc. (Neptune, N.J.); or Senflex T-Cell™ (Rogers Corp.,Rogers, Conn.). Examples of closed-cell foams of crosslinked copolymerscomprising polyethylene and ethylene vinyl acetate (EVA) include:Volara™ and Volextra™ (from Voltek Corp.); Senflex EVA™ (from RogersCorp.); and J-foam™ (from JMS Plastics JMS Plastics Supply, Inc.)

In other preferred embodiments, the closed-cell thermoplastic foamsubstrate 115 comprises a blend of crosslinked ethylene vinyl acetatecopolymer and a low density polyethylene copolymer (i.e., preferablybetween about 0.1 and about 0.3 gm/cc). In yet other advantageousembodiments, the blend has a ethylene vinyl acetate:polyethylene weightratio between about 1:9 and about 9:1. In certain preferred embodiments,the blend comprises EVA ranging from about 5 to about 45 wt %,preferably about 6 to about 25 wt % and more preferably about 12 toabout 24 wt %. Such blends are thought to be conducive to the desirableproduction of closed cells 140 of the thermoplastic foam substrate 115,having a small size (e.g., diameters between about 10 and about 500microns, and more preferably between about 50 to 150 microns). In stillmore preferred embodiments, the blend has an ethylene vinylacetate:polyethylene weight ratio between about 0.6:9.4 and about1.8:8.2. In even more preferred embodiments, the blend has an ethylenevinyl acetate:polyethylene weight ratio between about 0.6:9.4 and about1.2:8.8.

As further illustrated in FIG. 1, the thermoplastic foam substrate 115comprises closed cells 140. The term closed cell 140 as used herein,refers to any volume defined by a membrane within the substrate 115occupied by air, or other gases used as blowing agents, such as nitrogenor helium. The closed cells 140 form a substantially concave cell 135formed upon skiving of the substrate 115. The concave cells 135 need nothave smooth or curved walls, however. Rather, the concave cells 135 mayhave irregular shapes and sizes. Several factors, such as thecomposition of the thermoplastic foam substrate 115 and the procedureused to prepare the thermoplastic foam substrate 115, may affect theshape and size of the closed cells 140 and the concave cells 135.

As further illustrated in FIG. 1, the thermoplastic foam substrate 115can be coupled to an optional backing material 145. In some preferredembodiments the backing material 145 is stiff. A stiff backingadvantageously limits the compressibility and elongation of the foamduring polishing, which in turn, reduce erosion and dishing effectsduring metal polishing via CMP. In some cases, the backing material 145comprise a high density polyethylene (i.e., greater than about 0.98gm/cc), and more preferably a condensed high density polyethylene. Incertain cases, coupling to the thermoplastic foam substrate 115 isachieved via chemical bonding using a conventional adhesive 150, such asepoxy or other materials well known to those skilled in the art. In somepreferred embodiments, coupling is achieved via extrusion coating of themolten backing material 145 onto the thermoplastic foam substrate 115.In still other preferred embodiments, the backing material 145 isthermally welded to the thermoplastic foam substrate 115.

Another aspect of the present invention is a method for preparing apolishing pad for chemical mechanical polishing. FIGS. 2 to 4 presentselected steps in an exemplary method of preparing a polishing pad 200.Any of the embodiments of the polishing pad and its component parts,including the above described primary and secondary plasma reactants,can be incorporated into the method of preparing the polishing pad 200.

Turning now to FIG. 2, shown is the partially constructed polishing pad200 after exposing closed cells 210 within a thermoplastic foamsubstrate 220 of the polishing pad 200 to provide a substrate surface230 comprising concave cells 240. The concave cells 240 are formed onthe substrate's surface 230 by skiving. The term skiving as used hereinmeans any process to cut away a thin layer of the surface of thesubstrate 220 so as to expose concave cells 240 within the thermoplasticfoam substrate 220. Skiving may be achieved using any conventionaltechnique well-know to one of ordinary skill in the art.

FIGS. 3 and 4 illustrate selected stages in a surface coating process.Turning first to FIG. 3, illustrated is the partially completedpolishing pad 200 after exposing the substrate surface 230 to an initialplasma reactant, followed by exposure to a secondary plasma reactant ina PECVD process.

Exposure to the initial plasma reactant forms a modified surface 310 ofthe thermoplastic foam substrate 220. It is important to carefullycontrol the conditions and duration of the plasma treatment to avoidexcessive damage to the thermoplastic foam substrate 220. For example,an excessively high or uncontrolled radio flow discharge electrodetemperature can cause the thermoplastic foam substrate 220 to melt, warpor crack. In some preferred embodiments of the method, the radio flowdischarge electrode temperature is maintained between about 20° C. and100° C. and more preferably between about 30° C. and 50° C. In somecases an RF operating power between about 250 and about 1000 Watts, andmore preferably about between about 300 Watts and 400 Watts is used. Incertain preferred embodiments, the initial plasma reactant comprises aninert gas such as neon, and more preferably, argon or helium. In somecases, exposure to the initial plasma proceeds for between about 1second and 60 seconds, and more preferably about 30 seconds. In someembodiments, the PECVD reaction chamber is maintained at between about300 mTorr and about 400 mTorr, and more preferably about 350 mTorr.

FIG. 3 also shows the partially completed polishing pad 200 afterexposing the modified surface 310 to a secondary plasma reactant. Insome preferred embodiments the secondary plasma reactant comprisestetraethoxy silane (TEOS) or titanium alkoxide (TYZOR). In some cases,the secondary plasma reactant also includes the first plasma reactant,for example, TEOS or TYZOR vapor mixed with helium or argon gas.Exposure to the secondary plasma reactant results in the grafting of thesecondary plasma reactant to the modified surface 310 to form apolishing agent 320 comprising inorganic metal oxides. The polishingagent 320 coats an interior surface 330 of the concave cells 240.

Again, the conditions and duration of exposure to the secondary plasmareactant is carefully controlled to avoid damaging the thermoplasticfoam substrate 220, or the polishing agent 320, and to achievelong-lasting coatings of polishing agent 320. In some embodiments, thePECVD reaction chamber is maintained at between about 300 mTorr andabout 400 mTorr, and more preferably about 350 mTorr. In some instances,the radio flow discharge electrode temperature is maintained at betweenabout 20° C. and 100° C., and more preferably between about 30° C. and50° C. In some cases, an RF operating power between about 50 and about500 Watts, and more preferably, about 250 to about 350 Watts, is used.

Turning now to FIG. 4, illustrated is the partially completed polishingpad 200 after a period of exposure to the secondary plasma reactant ofat least about 30 minutes. In some preferred embodiments, exposure tothe secondary plasma reactant is for between about 30 minutes and about60 minutes. In other preferred embodiments, exposure to the secondaryplasma reactant is for between about 30 minutes and about 45 minutes.Such periods of exposure advantageously enhance the incorporation ofnitrides or carbides, or both, into the inorganic metal oxide of thepolishing agent 320. In some embodiments of the method, an interior ofclosed cells 410 of the thermoplastic foam substrate 220 comprisenitrogen gas. The nitrogen gas can react with the secondary plasmareactant to form nitrides. In other embodiments of the method, at leasta portion of the thermoplastic foam substrate 220 reacts with thesecondary plasma reactant to form carbides. For example, in someembodiments, carbon radical species within in an about 1 micron depth ofthe thermoplastic foam substrate 220 from the modified surface 310 canreact with the secondary plasma reactant.

As discussed above and further illustrated in the example section tofollow, the deposition of the polishing agent 320 via PECVD modifies thesurface properties of certain thermoplastic foams 230, such aspolyolefin foams. Surface coating of the thermoplastic foam surface 310with a polishing agent 320 for up to about 30 minutes occurs by onemechanism. After this period, however, surface coating occurs by adifferent mechanism. This, in turn, results in the production of apolishing pad surface 410 having distinct differences in surfacemicromechanics and chemistry, depending on the coating time.

In some cases, as the coating time increases, the temperaturethermoplastic foam substrate 220 temperature increases. This, in turn,causes out-gassing of the nitrogen gas used in foaming the substrate 220and located in the closed cells 410 of the substrate 240. In someembodiments, for instance where the polishing agent 320 comprisessilicon oxides, the out-gassed nitrogen reacts with the silicon specieson the pad surface, which causes formation of Si₃N₄ species instoichiometric conversion from SiO₄ to Si₃N₄. Of course, analogousreactions can occur in embodiments where the polishing agent comprisesother inorganic metal oxides such as titanium oxides.

Similarly, at long coating times, the ion bombardment of thethermoplastic foam substrate 240 surface 310 generates appreciableamounts of carbon radicals on the surface 310 of the pad 200. In someembodiments, for instance where the polishing agent 320 comprisessilicon oxides, these radicals react with the silicon species to formsilicon carbide (SiC), which are subsequently incorporated into thepolishing agent 320 coating the pad 200.

The incorporation of species such as Si₃N₄ and SiC into the polishingagent 320 modifies the polishing pad's 200 properties, such as enhancingits stiffness, hardness, and altering its modulus of elasticity, ascompared to the starting thermoplastic foam substrate or substratessubject to brief coating periods.

Having described the present invention, it is believed that the samewill become even more apparent by reference to the followingexperiments. It will be appreciated that the experiments are presentedsolely for the purpose of illustration and should not be construed aslimiting the invention. For example, although the experiments describedbelow may be carried out in a laboratory setting, one skilled in the artcould adjust specific numbers, dimensions and quantities up toappropriate values for a full-scale plant setting.

Experiments

Experiments were conducted to: 1) characterize the chemical compositionof thermoplastic foam substrates coated with polishing agents as afunction of coating time; 2) characterize the mechanical properties ofthe foam substrate coated with polishing agents; and 3) measure thepolishing properties of the polishing pads coated with polishing agentsas a function of coating time.

A thermoplastic foam substrate was formed into circular polishing padsof approximately 120 cm diameter of about 0.3 cm thickness. Thecommercially obtained thermoplastic foam substrate (J-foam from JMSPlastics, Neptune N.J.), designated as “J-60SE,” comprised a blend ofabout 18% EVA, about 16 to about 20% talc, and balance polyethylene andother additives, such as silicates, present in the commercially providedsubstrate. The J-60 sheets were skived with a commercial cutting blade(Model number D5100 K1, from Fecken-Kirfel, Aachen, Germany). The sheetswere then manually cleaned with an aqueous/isopropyl alcohol solution.

The J-60SE substrate was then coated with a polishing agent comprisingTetraethoxy Silane (TEOS), by placing the skived substrate into areaction chamber of a conventional commercial Radio Frequency GlowDischarge (RFGD) plasma reactor having a temperature controlledelectrode configuration (Model PE-2; Advanced Energy Systems, Medford,N.Y.). The plasma treatment of the substrate was commenced byintroducing the primary plasma reactant, Argon, for 30 seconds withinthe reaction chamber maintained at 350 mTorr. The electrode temperaturewas maintained at 30° C., and a RF operating power of 300 Watts wasused. Subsequently, the secondary reactant was introduced, for periodsranging from about 0 to about 45 minutes at 0.10 SLM, and comprisingTEOS mixed with He or Ar gas. The amount of secondary reactant in thegas stream was governed by the vapor back pressure (BP) of the secondaryreactant monomer at the monomer reservoir temperature (MRT; 50±10° C.).

The polishing properties of the J60SE polishing pads were examined bypolishing wafers having an about 4000 Angstrom thick tungsten surfaceand an underlying about 250 Angstrom thick tantalum barrier layer.Tungsten polishing properties were assessed using a commercial polisher(Product No. EP0222 from Ebara Technologies, Sacramento, Calif.). Unlessotherwise noted, the removal rate of tungsten polishing was assessedusing a down force of about 25 kPa of substrate, table speed of about100 to about 250 rpm (Product Number MSW2000, from Rodel, Newark Del.).A conventional slurry (Product Number MSW2000, from Rodel, Newark Del.)adjusted to a pH of about 2 was used.

FIG. 5 illustrates FTIR spectra of the substrate's surface afterdifferent periods of coating with TEOS. Spectra were obtained on a FTIRspectrometer (FTIR 1727, Perkin-Elmer System detector, equipped with aSeries-I FTIR Microscope (MCT detector) and having a spectral range from10,000 to 370 cm−1. Signals at about 1010 and about 950 cm−1 wereassigned to the asymmetric Si—O—Si stretch of silica and the Si—O—X(where X refers to polymeric —(Si—O—Si)n— structures not in thetetrahedral configuration), stretch of silicates, respectively. A signalat 850 cm−1 is due to free and associated silanols (Si—O—H). Thesilanols associate through hydrogen bonding with the extent ofassociation increases with increasing surface concentration of silanols.

As illustrated in FIG. 6, as coating time increases up to about 30minutes, both of these signals montonically decreased, due to a netdecrease in the surface concentration of Si—O Thereafter, there was achange in deposition kinetics and mechanism, indicating an increasedsurface Si—O concentration. The latter observation is inconsistent withgenerally accepted notions of TEOS deposition mechanisms and kinetics,and promoted further investigation of the coating process, especially inthe post-30 minutes coating period.

Nanoindentation testing was used to assess the mechanical properties ofthe surface coating coatings, and more specifically to measure theelastic modulus and the hardness. Indentations were carried out on thethermoplastic foam substrates coated with polishing agent for differentperiods. A NANOTEST 600®, Nanoindenter located at Advanced Material andCharacterization Facility (AMPAC, Orlando, Fla.) was used for allmeasurements. The machine rests upon a vibration isolation table and isenclosed in a temperature-controlled cabinet. Two separate heatersplaced on either side in front of the cabinet provide a thermal barrier.The temperature controller was set to a value about 2 or 3° C. above theroom temperature, with expected stability at ±0.1° C. The indenter wasallowed to settle for at least half an hour to attain thermal stabilitybefore starting the experiment.

Indentation parameters such as the type of indenter, the maximum depthand loading/unloading rate, were established by performing preliminarytests of the coated polishing pads. The polishing pad surfaces werefound to contain asperities and pores of varying sizes of the order offew microns. Based on these observations, a spherical indenter with tipdiameter of ˜1 mm was chosen, so that the indenter would sample enoughpad material. For the same reason, a spatial resolution of greater than500 microns was chosen. Indentations were performed under ultra low loadrange, with an initial load of 0.1 mN, which is a machine parameter. Thecontrol parameter was set to depth controlled and each pad was indentedfor varying depths with a maximum depth of 10,000 nanometers. Theresults were typically examined as an average of 10 indentations. BothOliver-Pharr method and Hertz method were used to evaluate and validatethe results. Load-depth (P-h) curves obtained from the nanoindenter wereanalyzed using the Oliver-Pharr method.

The polishing agent-coated polishing pads exhibited non-uniformpenetration during the indentation experiments. Visual examination ofthe indentation (P-h) curves shows some unique characteristics. As shownin FIG. 7, distinct events, labeled as ‘pop-in’ (Xb), ‘pop-out’ or‘kink-back’ (Xc), are discernable from the curves. For example, ‘pop-in’occurs during the compression cycle when there is a sudden penetrationof the indenter tip into the sample. These events correlate with severalexperimental parameters, such as coating time, loading/unloading rateand depth of indentation. The mixed response exhibited by the coatedpolishing pads revealed that ‘pop-in’ events occurred more often forindenter penetration depth around 1000 nanometers, ‘pop-out’ eventsseemed not to be affected by indentation depths, and rates ofloading/unloading were affected both of these events.

Further analysis of the P-h curve for various depths and differentcoating times, revealed that the loading curve increase steeply anddecreases, this X coordinates, or the depth of this transition point, isdesignated as Xa. Similarly, Xb and Xc are the corresponding Xcoordinates or depth in nanometers. Such non-uniform penetration of theindenter tip into the coatings probably results from the onset ofplastic deformation. It is thought that plastic deformation is acritical attribute of CMP pads, affecting the efficiency of the CMPprocess. Thus, the above-described events in the load-depth curves washypothesized to be predictors of pad performance. The initial surfacepenetration events (Xa) were found to be a function of PECVD coatingtime, maximum penetration depth and load rate. For instance, FIG. 8shows a representative correlation of Xa with the TEOS coating time. Thedata suggests that Xa is related to the thickness of surface foam thathas been modified by the dielectric coating.

FIGS. 9 and 10, respectively illustrate the hardness and elastic modulusof polishing pads for different TEOS coating times, calculated using theOliver-Pharr method. The effective surface modulus and hardness werefound to increase with increasing coating time. For coating times of 30minutes, 40 minutes and 45 minutes, the polishing pads had hardnessvalues of about 65 KPa, 62 KPa and 70 KPa, respectively. For shortercoating times, the hardness was 60 KPa or less. For coating times of 30minutes, 40 minutes and 45 minutes, the polishing pads had elasticmodulus values of about 4 MPa, 5.5 MPa and 8.2 MPa, respectively. Forshorter coating times, the elastic modulus values was 3 MPa or less.

The changes in the mechanical properties of the pad surface areattributed to the effect of the coatings deposited on the foamsubstrate. These data indicate that the previously noted discontinuitiesin the FTIR data are indicative of changes in the pad surface chemistry,rather than net removal of TEOS-derived coatings.

Dynamic mechanical analysis (DMA) was carried out on samples of coatedpolishing pads using commercial equipment operating in the tension modefrom −125 to 200° C. at a frequency of 1 Hz, at 10 micron amplitude witha programmed heating rate of 5° C./min. Liquid nitrogen was used toachieve the sub-ambient temperature. Samples were equilibrated at apredefined initial temperature for 10 minutes before measurements weremade. All of the polishing pad samples were prepared to have the samedimensions of 15 cm×5 cm, and were vacuum dried (30° C. at ˜1×10−2 Torr)for 24 hours prior to DMA measurements, so as to avoid having toconsider moisture effects.

As illustrated in FIG. 11, the DMA studies indicate an abrupt change inthe loss modulus at long PECVD coating times. At a coating time of 45minutes the loss modulus increase to about 0.6 MPa, as compares tovalues ranging from 0.37 to 0.23 for coating times of 10 minutes to 40minutes.

This is contrary to the generally accepted view that PECVD coatings onlymodify the surface of substrates. This surprising result suggests thatother processes occur that alter the bulk mechanical properties of thefoam substrate during the surface coating. It was hypothesized thatresidual reactants in the thermoplastic foam substrate were reactingduring PECVD coatings in a time dependent fashion.

The surface modification of polishing pads, subjected to differentcoating times, was further characterized using X-ray photoelectronspectroscopy (XPS). A commercial X-ray photoelectron spectrometer wasoperated at a base pressure of 10⁻¹⁰ Torr and the spectrometer wascalibrated using a metallic gold standard (Au (4_(f7/2)): 84.0±0.1 eV).A non-monochromatic Mg K ∝ X-ray source with an energy of 1253 eV at apower of 250 W, was used for the analysis. Charging shift produced bythe polishing pad samples were removed by using binding energy scalereferenced with respect to the binding energy of the hydrogen part ofadventitious carbon line at 285.0 eV. Peak deconvolution was carried outusing commercial software.

The XPS analysis provides several insights about the chemical nature ofthe topography resulting from TEOS adsorption a n d dissociation. TheCarbon (1s) signal was resolved into three major peaks: two peak at˜285.0 eV, corresponding to C—C and C—H bonds and peak observed at˜286.5 eV corresponding to C—O bonds. A peak centered at ˜289 to ˜289.3eV was attributed to carbamide [—O—C(NH₂)═O] functional group fromresidual blowing agents used in the thermoplastic foam substratemanufacturing process. For the specimens coated for 40 and 45 minutesrespectively, another peak near ˜283.6 eV was observed, and wastentatively assigned to C—Si bonds.

FIG. 12 presents exemplary peak fitted XPS signals of the Si (2p)envelop obtained from pads after TEOS coating times of: (a) 10 min, (b)20 min, (c) 30 min, (d) 40 min, and (e) 45 min. Peaks were identifiedas: (1) Si—O, (2) Silicate, (3) Si—N, and (4) Si—C bonds. Each spectrumwas deconvoluted into two major peaks at ˜102.3 and ˜103.4 eV,corresponding to bonds in silicate and Si—O species, respectively.

These data indicate that for short coating times (e.g., less than ˜30minutes) the pad surface is rich in silanol, consistent with TEOS filmsdeposited at low process temperatures. As further illustrated in FIG. 13the Oxygen to Si intensity ratio, calculated from the XPS data, is highearly during coating indicative of a high the concentration of thesilanol in the deposited coatings. The silanol concentration decreaseswith coating times up 30 minutes, then starts increasing. For example,as shown in FIG. 9, the O:Si ratio equals about 7.4, 4.8, 3.6, 7.1 and9.9, after coating times of about 10 min, 20 min, 30 min, 40 min and 45min, respectively.

Turning again to FIG. 12, for coating times of 30, 40 and 45 min, asmall peak is observed at ˜102.1 eV, corresponding to Si—N bonds. Overthis same period, there is an abrupt reduction in Si to N ratio, whichindicates an increase in nitrogen species on the surface.

These observations suggest that PECVD-based coating involvingcompetition between several processes. The PECVD-based coating producesboth silica and silicates on the (SiO_(x) and SiO₂) on the foam surface.Moreover, surface chemistry of the substrates changes as a function thecoating time. For coating times below 30 minutes, it there is netetching of the deposits from the Ar-ion bombardment of the surface. Thesample also heats up from the plasma, so thermal processes also occur.As the coating time increases, the silicate content on the substratesurface starts to decrease and the pad becomes denser, so as to increasethe hardness of the pad.

For coating times of 30 minutes of longer, the substrate temperature ishigh enough to cause out-gassing of the nitrogen gas used in foaming thesubstrate or decomposition of any residual blowing agent left in thefoam to produce nitrogen gas. The nitrogen reacts with Si-containingintermediates in the gas phase or Si-species on the pad surface, to formnitrides such as Si₃N₄ at the expense of SiO₂. Such nitrides areincorporated into the polishing agent in concentrations up to 10 mol %.Furthermore, for such coating times, the ion bombardment of the foamsurface generates appreciable amounts of Carbon radicals on the padsurface. These radicals react with the silicon species to form carbides,such as silicon carbide (SiC), which is incorporated into the polishingagent in concentrations up to 10 mol %.

FIG. 14 compares the Relative Blanket Tungsten Removal Rate (W-RR) andthe Static Coefficient of Friction (COF) for thermoplastic foamsubstrate subjected to different periods of coating times with TEOS.Both the W-RR and COF both increase with increased coating times up to30 minutes, signifying an increase in the thickness of the polishingagent. For coating times between 30 and 60 minutes, the W-RR and COFboth decrease, and then increase. These results suggest that the padappears polishes by one mechanism for surfaces coated for up to 30minutes, and by a different mechanism for surfaces coated for more than30 minutes, due to differences in surface micromechanics and chemistry.

Although the present invention has been described in detail, thoseskilled in the art should understand that they can make various changes,substitutions and alterations herein without departing from the scope ofthe invention.

1. A polishing pad for chemical mechanical polishing comprising: apolishing body comprising a thermoplastic foam substrate having asurface comprising concave cells; and a polishing agent coating aninterior surface of said concave cells, wherein said polishing agentcomprises an inorganic metal oxide that includes carbides or nitrides.2. The polishing pad as recited in claim 1, wherein said carbides ornitrides are incorporated into a lattice of said inorganic metal oxides.3. The polishing pad as recited in claim 2, wherein said nitridescomprise silicon nitrides or titanium nitrides.
 4. The polishing pad asrecited in claim 2, wherein said carbides include silicon carbides ortitanium carbides.
 5. The polishing pad as recited in claim 4, whereinsaid lattice of said inorganic metal oxides comprise silicon oxide ortitanium oxide.
 6. The polishing pad as recited in claim 1, wherein saidinorganic metal oxide comprise silicon carbides and silicon nitrides. 7.The polishing pad as recited in claim 1, wherein said nitrides compriseabout 10 mol percent of said polishing agent.
 8. The polishing pad asrecited in claim 1, wherein said carbides comprise about 10 mol percentof said polishing agent.
 9. The polishing pad as recited in claim 1,wherein said polishing agent has a oxygen to silicon ratio of at leastabout 8:1.
 10. The polishing pad as recited in claim 1, wherein saidpolishing pad has a hardness of about 60 KPa or greater.
 11. Thepolishing pad as recited in claim 1, wherein said polishing pad has anelastic modulus of greater than about 3 MPa.
 12. The polishing pad asrecited in claim 1, wherein said polishing pad has an loss modulus ofabout 0.4 MPa or greater.
 13. A method for preparing a polishing pad forchemical mechanical polishing, comprising: exposing closed cells withina thermoplastic foam substrate to provide a substrate surface comprisingconcave cells; and coating an interior surface of said concave cellswith a polishing agent comprising an inorganic metal oxide, whereincarbides and nitrides are incorporated into said inorganic metal oxideduring said coating.
 14. The method as recited in claim 13, whereincoating comprises: exposing said substrate surface to an initial plasmareactant in a plasma enhanced chemical vapor deposition (PECVD) processto produce a modified surface thereon; and exposing said modifiedsurface to a secondary plasma reactant in said PECVD process to formsaid polishing agent.
 15. The method as recited in claim 14, whereinsaid initial plasma reactant comprises argon or helium, and exposure tosaid initial plasma proceeds for about 30 seconds.
 16. The method asrecited in claim 14, wherein exposure to a secondary plasma reactantproceeds for at least about 30 minutes.
 17. The method as recited inclaim 14, wherein exposure to a secondary plasma reactant proceeds forbetween about 30 minutes and about 60 minutes.
 18. The method as recitedin claim 14, wherein said secondary plasma reactant comprisestetraethoxy silane or titanium alkoxide.
 19. The method as recited inclaim 14, wherein an interior of said closed cells comprise nitrogen gasand said nitrogen gas reacts with said secondary plasma reactant to formsaid nitride.
 20. The method as recited in claim 14, wherein saidthermoplastic foam substrate reacts with said secondary plasma reactantto form said carbide.